My Beretta 1680-1980 O/U Muzzeloader fires unreliably, especially from the botton barrel. The misfires aren't a function of cleaning. Had a gunsmith, flush the barrels. I understand that this is common problem with this gun and that the problem relates to soft easily corroded metal used in the flame channel. I recall once reading that it was possible to replace the flame channels but I haven't been able to find where I read about the procedures. Anyone have experience with this problem or with the fix?